1. Field of Invention
The invention relates to preform cutting elements for rotary drag-type drill bits, for use in drilling or coring holes in subsurface formations, and of the kind comprising a bit body having a shank for connection to a drill string, a plurality of cutting elements mounted at the surface of the bit body, and a passage in the bit body for supplying drilling fluid to the surface of the bit body for cooling and/or cleaning the cutters. Each cutting element comprises a front facing table of superhard material bonded to a less hard substrate.
2. Description of Related Art
The cutting element may be mounted on a carrier, also of a material which is less hard than the superhard material, which is mounted on the body of the drill bit, for example, is secured within a socket on the bit body. Alternatively, the cutting element may be mounted directly on the bit body, for example the substrate may be of sufficient axial length that it may itself be secured within a socket on the bit body.
In drag-type drill bits of this kind the bit body may be machined from metal, usually steel, and sockets to receive the carriers or the cutting elements themselves are machined in the bit body. Alternatively, the bit body may be moulded from tungsten carbide matrix material using a powder metallurgy process. Drag-type drill bits of this kind are particularly suitable for drilling softer formations. However, when drilling soft, sticky shale formations in a water based mud environment, and in other similar conditions, there may be a tendency for the shavings or chips of formation gouged from the surface of the borehole not to separate from the surface and to be held down on the surface of the formation by the subsequent passage over the shaving or chip of other cutters and parts of the drill bit. Also, there may be a tendency for such material to adhere to the surface of the bit body, a phenomenon known as "bit balling", eventually resulting in the bit becoming ineffective for further drilling.
In order to alleviate or overcome this problem, the facing table may be formed with a chip breaker which serves to break the shaving or chip of formation into fragments as it passes over the front surface of the cutting element, thus enabling the particles to be entrained in the flow of drilling fluid, and swept away from the cutting element, so that they are not held down on the formation or do not adhere to the bit.
The present invention sets out to provide improved forms of chip breakers for preform cutting elements for rotary drag-type drill bits.